a) Field of the Invention
The present invention relates to a method and device for producing extreme ultraviolet radiation (EUV) or soft X-ray radiation.
A preferred field of use of the present invention includes applications that require soft X-ray light, i.e. EUV light, in the 1-20 nm spectral range. The most prominent application is EUV projection lithography with an operating wavelength of 13.5 nm where compact, powerful, cost-efficient and reliable light sources are required. An additional field of applications includes X-ray analytic methods such as photo electron spectroscopy or fluoro-X-ray analysis which utilize the spectral range of soft X-ray radiation and which can be realized on a laboratory scale. Furthermore, the method and device can be utilized for the characterization of X-ray optics or X-ray detectors and finally as a source for an EUV microscope in the spectral range of the so-called water window for in vivo observation of biological tissues.
b) Description of the Related Art
The use of a plasma as a source for EUV light and soft and hard X-rays is well known. Nearly independent from the method of plasma generation, the emitting plasma has to be sufficiently hot (i.e. >150,000° K) and dense (i.e. >1017 electrons/cm3) to emit X-rays and/or EUV radiation.
Different techniques for producing EUV radiation are known that fulfill the above conditions. They can be divided into discharge based or laser based plasma source concepts.
For so-called gas discharge produced plasma (GDPP) sources, a pulsed discharge generates a “spark-like” plasma with currents of some 5 to 100 kA flowing through the plasma for times of some 10 nanoseconds to some microseconds. For increasing the conversion to EUV by additional heating and compression, the so-called pinch effect might contribute to the process. The different concepts of discharge plasmas differ in electrode geometry, voltage-pressure range, plasma dynamics, ignition strategies and in the electrical generator. Various examples of such discharge plasmas are known such as dense plasma focus Z-pinch discharge, capillary discharges and hollow cathode triggered pinch. Different versions of such discharge plasma concepts are disclosed in patent documents U.S. Pat. Nos. 6,389,106, 6,064,072 and WO 99/34395.
For so-called laser produced plasmas (LPP) a laser beam is focused to some dense (>1019 atoms/cm3) matter (most frequently called target). If intensities exceed some 1010 W/cm2 EUV or even X-ray radiation is emitted from nearly any material. Various concepts using laser irradiated targets for plasma generation have been disclosed in patent documents WO02/085080, WO02/32197, WO 01/30122 and U.S. Pat. No. 5,577,092.
With common state of the art source concepts having maximum conversion efficiencies between 0.5 and 2%, typically 50,000 W to 100,000 W excitation power have to be coupled into the emitting plasma in order to obtain sufficient useful EUV power (80-120 W) for industrial applications such as EUV lithography. This translates into generation of 300 W up to more than 1,000 W of EUR radiation directly at the source spot, depending on the source concept. For the existing source concepts LPP and GDPP, several factors make it extremely difficult to satisfy these required EUV power levels:
1/For the LPP concepts, the limitation will be by two factors: First, it is expected that the costs of a laser with some 10 kW of power will by far exceed the budgets which are defined by economic production costs. Second, the electrical power needed to drive the laser (typically about one MW) and the required cooling will likely exceed acceptable scale at semiconductor factories.2/For the GDPP concepts, the limitation is as follows. The power has to be fed into a volume of typically 103 times the volume which emits the radiation. For a tolerable source volume of 1 mm3 the typical discharge volume is of 1 cm3. As the confinement of this volume is traditionally accomplished by either the discharge electrodes or by insulator material, these materials are heavily heated and eroded, because their typical distance from the hot plasma is allowed to be only in the order of some millimeters to centimeters.
Thus both laser produced plasma (LPP) and gas discharge produced plasma (GDPP) appear to be un-adapted to the latest requirements for industrial applications, in particular for extreme ultraviolet radiation lithography (EUVL). Consequently, an urgent demand for novel technical solutions arises which appears to be a condition sine qua non for the successful introduction of EUVL following the IRTS roadmap (2009) and Intel roadmap (2007).